Current limiting fuse



Jan. 13, 1970 s. R. SMITH, JR 3,489,977

Y CURRENT LIMITING FUSE Filed Feb. 15, 1968 United States Patent3,489,977 CURRENT LIMITING FUSE Sidney R. Smith. Jr., Myrtle Beach,S.'C., assignor to General Electric Company, a corporation of New YorkFiled Feb. 15, 1968, Ser. No. 705,798 Int. Cl. H01h 71/20, 73/22 U.S.Cl. 337-146 10 Claims ABSTRACT OF THE DISCLOSURE The present inventionrelates to electrical fuses and more particularly to an improved fuselink which is exceptionally well adapted for use in providing overloadcurrent protection for electric power distribution systems thatvareoften subjected to both sustained relatively lowcurrent overloadconditions as well as to occasional highcurrent overload surges.

Current limiting fuses now commonly used for the protection of powerdistribution system components are usually designed to provideprotection from either high current surges that must be interruptedimmediately, or relatively small overloads that need only be interruptedif they are sustained for a substantial length of time at a currentlevel that might generate enough heat to damage certain components ofthe system. It is also fairly common practice to provide such systemswith fuses that afford both of these current limiting functions in asingle fuse cartridge having a suitable series connected arrangementtherein. However, the prior art series connected dual-element fusespresently available generally incorporate a fairly complex combinationof fuse elements, each having different time-current characteristics,and often requiring the further combination of some type of heatingarrangement for one of the elements. In addition to `their complexityand resulting expense, such arrangements have several operationalshortcomings. For example, a common problem in such fuses is that inorder to obtain rapid burn back of the low overcurrent element to assureclearing of the circuit when this element melts, it is necessary to makethe separate fuse wires of very small diameter, which makes the wiredicult to work with during the manufacture of the fuse. A secondinherent problem with such fuses has been that heretofore they have notbeen capable of accurately and consistently providing overloadprotection for very low values of sustained overcurrent.

A further problem encountered in prior art fuses for overload protectionstems from the fact that frequent temperature cycling of the fuseelement during its operational life often results in the relativelyweak, reduced area portion of the fuse being broken as it is repeatedlyforced to expand and contract. My invention overcomes the foregoingdisadvantages of prior art fuse devices while atthe same time affordinga simple and low cost fuse construction.

Accordingly an object of my invention is to provide an improved fusethat is adapted for use in power distribution systems and which willprotect the components of such systems from damage due to either shortduration high current transients or sustained relatively low overcurrentconditions in the system.

3,489,977 Patented Jan. 13, 1970 ICC Another object of the invention isto provide an improved fuse having a time-current characteristic whichprovides a relatively slow, low overcurrent limiting function.

A further object of the invention is to provide an improved fuseconstruction in which the fusible elements employed will not besubjected to the hazard of breakage during their normally longoperational life in which the fuse elements are repeatedly heated andcooled.

Further objects and advantages will become apparent as the descriptionproceeds and the features of novelty which characterize my inventionwill be pointed out with particularity in the claims annexed to andforming a part of the specification.

In the drawing,

FIG. l is a side elevation, partly in cross section, of a currentlimiting fuse embodying a preferred form of my invention.

FIG. 2 is a side elevation, partly in cross section, of a fuse embodyinga second from of my invention.

FIG. 3 is a side elevation, partly in cross section of anotherembodiment of a fuse link constructed pursuant to the teachings of myinvention.

Referring now to FIG. l of the drawing, there is shown a currentlimiting fuse illustrated as comprising a tubular casing 1 of insulatingmaterial, such as pressed organic fiber, glass or ceramic, upon theopposite ends of which are mounted metal terminals 2 and 2. Theterminals 2 and 2 may be sealed to the tubular casing 1 in anyconventional manner, such as by sealing them hermetically with asuitable epoxy glue. A granular arc quenching material 3, such as sand,is enclosed within the envelope formed by the tubular casing 1 andterminals 2 and 2. A first plurality of fusible conductive elements 4,which comprise silver wires of equal and substantially uniform diameterin the preferred embodiment of my invention, are wound in helical formaround a cylindrical member 5, and are each electrically connected atone end to the terminal 2. The member 5 is formed of a nonlinearresistance material such as silicon carbide of the type commonly used invalve-type lightning arresters. The other ends of the fusible wires 4are twisted to form a junction 6 with the respective ends of a secondplurality of elongated fusible elements 7, which also comprise silverwires of substantially equal and uniform diameter in the preferredembodiment of my invention. The other ends of the silver wires 7 areelectrically connected, respectively, to the terminal member 2 by beingsoldered thereto. A circular disc 8 formed of any suitable insulatingmaterial, such as glass, and having a plurality of equally spacedperipheral notches 8a therein, is disposed within the tubular casing 1to help maintain the fusible conductors 7 in spaced relationship over amajor portion of their length. As shown in FIG. l, each of the wires 7is positioned in one of the notches 8a. It will be understood thatfusible conductors 4 and 7 may be formed of other suitable eutecticmaterial, such as a silver-copper alloy, and my invention is not limitedto the use of silver wire for these elements.

In order to provide a fusible arrangement that will immediatelyinterrupt a high-current overload without producing an unduly severevoltage kick, the combined diametrical cross-section of the fusibleconductors 4 is substantially smaller than the combined diametricalcrosssection of the fusible conductors 7. Accordingly, when a highovercurrent surge exists in the fuse, the fusible elements 4 immediatelyvaporize causing an arc to be formed between terminal 2 and the junction6, along the length of the nonlinear resistance 5. The nonlinearresistance member S is thus placed in parallel with the resultant arcand, accordingly, conducts a portion of the current that flows as thefusible elements 7 are burned back; therefore, the voltage kick thatwould otherwise be transmitted to the system when the elements 4 fuse,is substantially reduced, by the simultaneous introduction of thenonlinear resistance member 5 into the circuit where wires 4 vaporize.

If the fuse device is subject to a low overload current condition ofextended duration, the juxtaposed portion of fusible elements 4 and 7formed at the twisted junction 6 will fuse and rupture, due partially tothe fact that this novel arrangement serves to concentrate the heatgenerated by currents flowing through the respective conductors 4 and 7at the junction 6. For the major portion of their respective lengths,the fusible elements 4 and 7 are surrounded by the arc quenching sand 3,therefore, over this portion of their lengths they are maintained in arelatively cool state by the heat-sink effect of the sand 3 so they willnot heat as rapidly as thev junction 6 during sustained low currentoverload conditions. To further accelerate the rate of fusion of thejunction 6, in the preferred embodiment of my invention, the twistedconductors at 6, are bonded together with a bead of eutectic material 9comprising 25% cadimum, 50% tin and 25 lead. I have found this to be aparticularly desirable eutectic alloy for the purposes of my inventionbecause it melts at a much lower temperature than the silver wires 4 and7, and causes these elements to form a low melting point alloy thatfuses and ruptures at a temperature substantially below the meltingpoint of pure silver. In addition, the alloy thus formed with silverwires 4 and 7 has a higher electrical resistance than pure silver. Thisformation of the alloy both lowers the melting point of the fusiblewires 4 and 7, and simultaneously increases the amount of heat generatedat the junction 6. This operating characteristic of my invention is alsoimproved by providing an insulating sheath 10 around the twistedjunction 6 to confine the heat generated at the junction during asustained overload condition. Sheath 10 may be formed of a ber cylinder,as shown, or other suitable heat insulating means, such as insulatingtape wrapped around the junction 6.

While it is known in the prior art to use cadmium-containing beads onfuse elements to lower their temperature of fusion, the above mentionedalloy has been found to be particularly desirable in practicing myinvention because of the desirably low melting characteristics itprovides. For example, with the structure disclosed it has been foundthat the melting point of the fusible elements at junction 6 can belowered 50% below the melting temperature of the spaced apart,uninsulated portion of the fusible elements 4 and 7.

To further illustrate the features of my invention, reference is nowmade to FIG. 2 of the drawing wherein a modification of the invention isdepicted. In FIG. 2, the component parts of the illustrated devicesimilar to those in FIG. 1 will be designated with like referencenumerals. Accordingly, there is shown a tubular casing 1 havingelectrically conductive terminals 2 and 2' sealed in a suitable mannerat the opposite ends thereof. A first plurality of conductive fusibleelements 4 are electrically connected at one of their respective ends tothe terminal member 2 while a second plurality of fusible elements 7 areconnected in a similar manner to the terminal element 2'. Circular,notched, insulating discs 8 are disposed in the tubular casing 1 betweenthe respective elements 4 and 7 with the notches 8a each positioning oneof the fusible elements to maintain it in spaced apart relationship withrespect to the remainder of the elements. A twisted junction 6 is formedbetween the respective other ends of the fusible conductors 4 and 7 andthis junction is encased in a heat insulating wrapping 10, such asinsulating tape, that serves to maintain the heat generated at thejunction in thermal proximity therewith. The interior of the envelopeformed by the casing 1 and the terminals 2 and 2 is filled with agranular insulating material, such as the sand 3. This embodiment of theinvention functions in much the same manner as the embodimentillustrated in FIG. 1; namely, the combined cross section of theplurality of fusible elements 4 is of smaller diameter than that of thefusible elements 7 and, thus, the elements 4 vaporize first when a highovercurrent surge is passed between the terminals 2 and 2. It will benoted that the nonlinear resistance element 5 which was present in thepreferred embodiment of FIG. 1, is not employed in this embodiment ofthe invention. This omission of the nonlinear resistance causes agreater voltage kick to be induced `when the elements 4 fuse, but theheavier cross lsection of elements 7 causes them to burn back moreslowly to thus reduce the magnitude lof the voltage kick somewhat. In asimilar manner, it will be apparent that the insulating spacing discs 8may be removed without departing from the scope of the invention;however, in order to provide for desirable radiant and conductivecooling of the fusible elements 4 and 7, I prefer to maintain them insubstantially equidistant spacing and, therefore, utilize the discs 8 inthe illustrated embodiment.

An important feature of my invention resides in the fact that the lowcurrent fusible portion of the junction 6 is mechanically strong due tothe fact that its mechanical cross section is greater than themechanical cross section of the respective fusible conductors 4 and 7that feed into it. Accordingly, unlike other prior art fuse devices thatprovide a low overcurrent fusing function by reducing the mechanicalcross section of the fuse element, and thus weaken the element at thatpoint, the fuse of my invention can withstand repeated temperaturecycling with the resultant expansion and contraction of the fusibleelement, without any danger of rupture due to mechanical failure at thelow overcurrent fusible junction 6.

In both of the embodiments of my invention illustrated in FIGS. l and 2,the fusible elements 4 and 7, on opposite sides of the respectivejunctions 6, comprise separate elements. It should be understood fromthe foregoing discussion that the fusible elements 4 and 7 could be;formed of different materials, shaped to different respective diameters,or varied in number, so that one of the groups of elements consistentlyinitiates fusion when a high overload current is passed through thefuse, while the other group of elements serves to reduce voltage kick byaffording a slower rate of burn back. Although the above describedembodiments of my invention are its more preferred forms, yet anothermodification of my invention `affords a simplified,inexpensive-to-manufacture structure that also possesses the desirableoperating characteristics of my invention.

Referring to FIG. 3, this modication of my invention is shown in theform of a fuse link `comprising a plurality of silver Wires 4' that aremaintained in a spaced-apart relationship for a major portion of theirlengths, as in zones A and B, and held in juxtaposition for a shortportion of their lengths, as in zone C. It will be understood that otherconventional fusible elements than the silver wires embodied in thisform of the invention can be used without departing from the scope ofthe invention. For example, a plurality of fusible ribbons or bandsformed of an eutectic alloy might be employed in other embodiments ofthe invention. Of course, any suitable spacing means, such as thoseemployed in the modifications depicted in FIGS. l and 2 in the form ofdiscs 8, may be used to maintain the substantially equidistant spacingof wires 4' in zones A and B; therefore, no specific spacing means isillustrated in FIG. 3. In the zone C, a heat generating junction 6 isformed by the closely spaced wires 4. This junction `6 retards heatdissipation to the surrounding environment, due to the relatively smallsurface area of the wires 4 in contact with that environment, asexplained above with regard to the modifications of the invention shownin FIGS. 1 and 2. Also, an insulating housing 10" is disposed around thejunction 6 and a band of cadmium alloy 9, comprising 25% cadmium, 50%tin and 25% lead is deposited around the junction 6. In addition toproviding a low melting point eutectic alloying material at junction 6',the band of alloy 9 serves to hold the wires 4 together prior to thetime housing 10 is put in place. It will be noted that a significantfeature of this form of the invention is that the fusible elements 4 arecontinuous over their respective lengths, including the portion thereofforming the junction 6'; therefore, it will be appreciated that such afuse link is easily fabricated by simply twisting a plurality of suchelements 4' to form a juncton 6', then depositing a band of solder 9around the junction and encasing it is an insulating housing 10'.

In operation, the modification of the invention illustrated in FIG. 3functions -as follows: when a low overcurrent is passed through the fusefor a predetermined sustained length of time, enough -heat is generatedin housing 10 by the juxtaposed wires 4' at junction 6 t0 melt thecadmium alloy band 9. The melted alloy band then forms an alloy with thewires at junction 6. This alloy has a low fusing point and relativelyhigh resistance which combines to cause the wires to fuse at atemperature level approximately 50% below the temperature at which theportions of wires 4', in zones A and B, will fuse. Accordingly, an arcis formed at the midpoint of junction 6' and it burns back the fusibleelements 4 to effect an interruption of any circuit in which the fuselink may be connected.

A significant difference exists between the high overcurrent operatingcharacteristics of this form of my invention and those of the formsillustrated in FIGS. 1 and 2. Since the cross sectional area of t-hecurrent conducting fusible elements 4 remains unchanged over the entirelength of the fuse link, in the FIG. 3 embodiment, a high current surgevaporizes the portions of wires 4' in both zones A and B, so only thethick junction 6 serves to retard the rate of burn back.

While I have shown and described particular embodiments of my invention,it will be obvious to those skilled in the art that changes andmodifications may be made therein without departing from the invention,and therefore, it is intended by the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A current limiting fusible device comprising first and second spacedapart terminals, a first plurality of fusible conductors each connectedat one of their ends to said first terminal, a second plurality offusible conductors each connected at one of their ends to said secondterminal, said first and second plurality of conductors being arrangedto maintain a substantial lateral spacing between their respectiveconductors for a major portion of their lengths, means forming anelectrical junction between the respective other ends of said first andsecond pluralities of fusible conductors, said first and secondpluralities of conductors being arranged in juxtaposition for a shortportion of their respective lengths adjacent the point of theirelectrical junction, the timecurrent characteristic of said firstplurality of fusible conductors being substantially faster than thetime-current characteristic of said second plurality of fusibleconductors whereby said first plurality of conductors ruptures firstwhen a high overcurrent is passed through the device.

2. A current limiting fusible device as defined in claim 1 wherein saidmeans forming an electrical junction cornprises `a layer of eutecticmaterial bonded around the respective other ends of said first and saidsecond pluralities of fusible conductors.

3. A current limiting fusible device defined in claim 2 wherein saidother ends of said first and second pluralities of fusible conductorsare twisted together for a relatively short portion of their overallrespective lengths.

4. A current limiting fusible device as defined in claim 1 including abody of granular arc quenching material disposed between said first andsecond terminals and around all of said fusible conductors.

5. A current limiting fusible device as defined in claim 3 including aninsulating material disposed around the twisted junction of saidconductors, said material being effective to thermally insulate saidconnection.

6. A fuse including first and second spaced apart terminals, a tubularinsulating casing disposed between said terminals and sealed thereto toform an envelope, a first plurality of fusible conductors each connectedat one of their ends to said first terminal, a second plurality offusible conductors each connected at one of their ends to said secondterminal, an electrically conductive nonlinear resistance memberdisposed within said envelope, each conductor of said rst plurality ofconductors being tightly wrapped on said member and arranged to maintaina substantial lateral spacing respectively therebetween said conductorsform a plurality of parallel hellical paths around said member, meansforming an electrical connection between the respective other ends ofsaid first and second pluralities of fusible conductors, said first andsecond pluralities of conductors being arranged in heat exchangingjuxtaposition for a short portion of their respective lengths adjacentthe point of their electrical connection.

7. A fuse as defined in claim 6 wherein said means forming an electricalconnection comprises a solder containing 50% tin, 25% lead and 25cadmium mutually bonded to the respective other ends of said first andsecond pluralities of fusible conductors.

8. A fuse as defined in claim 6 including a body of granulated arcquenching material disposed in said envelope around said bonded solderconnection.

9. A fuse as defined in claim 2 wherein said layer of eutectic materialcontains more than 10% by weight of cadmium.

10. A fuse as defined in claim 2 wherein said layer of eutectic materialcomprises an alloy of approximately 50% tin, 25 lead and 25 cadmium.

References Cited UNITED STATES PATENTS 1,692,445 11/1928 Hope 337-2912,876,312 3/1959 Frederick 337-163 X FOREIGN PATENTS 1,021,069 2/ 1966Great Britain.

BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, Assistant ExaminerU.S. Cl. X.R. 337-163, 291

fjg" UNHED STATES PATENT of CERTIFICATE OF CORRECTION Patent No.3,489,977 Dated January I3, |970 Invencor(s) Sidney R. Smith, Jr

It is eettified that error appears in the above-identified patent andthat said Lettere Patent are hereby corrected as shown below:

Cobumn 6, I inefl -4- Si gned and sealed this 9th day of Maly` 1970(SEAL) Attest:

EDWARD IWFLETCHERJR. WILLIAM E. SCIIUYLER, JR.

Commissioner of Patents Attesting Officer

